A gas meter, comprising a meter housing having a gas inlet, a gas outlet and a measuring section, in which a measuring device, in particular a measuring device for thermal flow measurement, is arranged, wherein a filtering device for removing particulate impurities is arranged downstream of the gas inlet and wherein the filtering device is supplied with and purifies only a gas portion which is diverted from the main gas flow, after which the purified gas portion is fed to the measuring device arranged downstream of the filtering device.
Using the measuring device integrated into them, gas meters are used to determine the quantity of flowing gas which enters via the gas inlet in the housing and passes into the measuring section, where the flow is detected by means of the measuring device, after which the gas reemerges via the gas outlet in the housing. In modern gas meters, thermal flow meters, sometimes also referred to as microthermal sensors, are used as measuring devices. This is an electronic flow meter operating on a thermal or calorimetric measuring principle. A sensor of this kind comprises a heating element and two temperature sensors, of which one is arranged upstream of the heating element and the other is arranged downstream of the heating element, when viewed in the direction of flow. There is a temperature difference measured between the two temperature sensors, depending on the flow velocity of the gas flowing past and hence depending on the flowing gas quantity, and this difference is evaluated in order to determine the flow rate. The principle of a thermal flow sensor of this kind is known.
To ensure that a flow sensor of this kind, which normally delivers highly accurate measured values, actually operates with corresponding precision, it is necessary to keep any contaminants away from the sensor since such contaminants have a negative effect on the result of measurement. For this purpose, it is a known practice with known gas meters of this kind to precipitate particulate impurities, i.e. dust particles and the like entrained in the gas, by means of a filtering device arranged downstream of the gas inlet and upstream of the measuring section or measuring device, e.g. by means of a filter element or a particulate trap, e.g. a cyclone dust separator or the like. However, there is the problem here that very small particles and fine dust can be removed only with difficulty by means of fluid-mechanical filtering devices, for example, and therefore the filtering device has to be of very complex design to remove even the finest particles. This is because such extremely high purity of the gas is ultimately necessary since even particles with a size of 5 μm can impair the accuracy of measurement of the microthermal flow meter. That is to say, such sensors are not sufficiently robust with respect to such impurities. However, if correspondingly complex filters are used, the resulting pressure loss between the region upstream and the region downstream of the filtering device is too high, and this is likewise not desirable, especially since there is the risk that such a filtering device will become blocked.
US 2010/0175468 A1 discloses a fluid flow meter in which there is, upstream of an orifice plate, a flow branch, which is passed through a filtering device and a measuring device and returned back into the main flow duct downstream of the orifice plate.
In the gas meter according to U.S. Pat. No. 4,381,668, a part of the gas flow upstream of a tube constriction is passed through a filter element into a measuring section and is then passed back into the main flow at the level of the tube constriction.
DE 39 05 746 A1 and DE 39 22 489 A1 disclose air meters in which a part of the air flow is passed through a bypass duct, in which it is filtered and then its flow rate is measured.
Dividing a flow to be measured into a main flow and one or more measurement flows with filtering in the partial flow is also known from US 2009/0188303 A1 and DE 36 36 930 A1.
The problem underlying the invention is to specify a gas meter which allows very good purification of the gas fed to the measuring device while simultaneously avoiding an excessive pressure loss due to the purification.